Vacuum valves



United States Patent 72] inventor Ivor John Taylor PrimaryExaminer-William F. ODea 560 Riverside Drive apt. 60, New York,Assistant Examiner-David R. Matthews New York 10027 [21] Appl. No.738,434 4 [22] Filed June 20, 1968 [45] Patented Nov. 17,1970 7ABSTRACT: The invention relates to fast acting valves suitable for usein high vacuum systems. The valves employ a [54] VACUUM VALVES pistonelement which reciprocates in the cylindrical bore of 13 Claims, 12Drawing Figs. the valve casing. in one valve type the piston forms ahigh 521 U.S. Cl. 251/259, g vacuum t" f 29 g? f: 251/284, 251/324,251/334, 251/357, 251/367 y u 112mg a narrow sefc ion-sea ilng mem er reame inl e [51] int. Cl Fl6k 31/528 a W a formed within a detachable endsleeve withdrawable from the [50] Field otSearch 251/251, valve casing,the 5|eeve construction enabling high quality vacuum sealing underconditions of extensive usage by reduc- 367; 137/434 tion of frictionalwear on the sealing member. Reciprocal motion of the piston is obtainedfrom a rotary movement of an ac- [56] References Cned tuating spindlewith crank disc. The crank disc is located UNITED STATES PATENTS withina bored recess in the valve casing to allow absorption of 1 1,467,816 /12 Rope 251/284X operating stresses by the casing and permit the use ofan effi- 1,753.419 5/1 30 Wh awnm- 251/259 cient vacuum sealingarrangement for the spindle. A crank pin 5 ,560 10/1953 Smith 25 l/36ZXand external stops arrangement enables the valves to be 2,690,322 9/l954Stansfield 251/324 Iogked in their open and closed positions.

' 3 i /5 a a 53 L3 35c I 27 I /2 i 9 23 l f5 Patented Nov. 17, 1970 Sheet Patented Nov. 17, 1970 Sheet FIG.2

Patented Nov. 17, 1970 3,540,695

Sheet .of 4

Patented Nov. 17, 1970 Sheet 1- of 4 4 a 2 3 f 3 i a 7 2m 2 ,2 0 Q 4fill iii 6\ v P a y If I. ...l a /H i U l 8 a 6 6 W-w fllaall w .mwl l 0A M Q N Z M J W ,7 w 9 y M Fla/2 l VACUUM VALVES This invention relatesto valves and to that field of valves hereafter referred to as vacuumvalves which are suitable for use in high-vacuum systems.

Objects ofthe valve include the devising of 1. A valve which issufficiently leak free to be capable of sustained operation inhigh-vacuum systems,

2. A valve having a quick and single-movement operation- .this featurebeing intended to anticipate and meet the requirement of making manyrepeated cycles of operation (which may be involved, for instance, inthe production line vacuum testing of various products).

. A valve which is locked when in the closed or open positions so thatthe valve cannot be opened or closed as a result of pressuredifferentials across the valve ports.

4. A valve havin g a single mechanism of but one rotating and onesliding part and which, by virtue of its construction, is capable ofeffecting a firm and locked vacuum seal yet retaining the feature ofeasy release.

. A valve whose sealing and release is obtained without the need ofpressure springs or other such devices and whose locking action obviatesthe necessity of any complicated locking mechanism or indeed of anyextra mechanism devoted to the object of locking the valve.

A valve having large port apertures (to achieve high pumping speedthrough the valve) together with a small size and compact form for thevalve unit.

. A valve whose mechanism can be readily removed from the valve body forservicing or replacement without removing the valve body from the vacuumsystem, into which it may therefore be permanently installed.

As is' well known, valves which are fully suitable for fluid control arein general wholly unsuitable for use in highvacuum systems due toinadequacies in the firmness of sealing at the valve ports-and, mostoften, because of inadequate sealing of the valve body or the valveoperating mechanism against external leakages into the valve.

A valve for high-vacuum'use has to be substantially leakfree. Forexample, a leakage totaling lcc. of air over a whole day of operationcorresponds to an equivalent leakageof 760 vacuum or, in other terms,corresponds to a'lead rate of more than 7,600 cc per second at thispressure of vacuum. Such an enormous leakage could completely overwhelma vacuum system attempting to produce and maintain this degree Torr)ofvacuum.

Modern leak-detection methods at present enable leaks of the order ofl0- atmospheric cc. per sec. to be detected and a valve suitable for usein high-vacuum work necessarily must be leak-free in operation to atleast this extent and which is the capability of the valve hereafterdescribed. Vacuum valves therefore require the following features:

a. that they provide a high compressionsealing within the valve toisolate the valve ports; and

b. that they use shaft or stem seals which are specifically,

and often elaborately, designed to avoid leakage into the valve.

in a reciprocating piston type of valve, which is perhaps the simplestvalve concept, the piston is generally operated by the shaft momentarilybreaks-away as translational motion is started through the seal; 7

In general, therefore, valves utilizing a translational movement of ashaft through a seal are basically unsuitable for use as vacuum valves.The contention that such valves might be made satisfactorily leak-freefor many vacuum uses rests on minimizing the two effects mentionedabove.

T hus, necessarily, the shaft movement should be a small one, and oneinvolving a part of the shaft that is well polished and protected toremain clean and scratch free. Any dust or dirt that may accumulate on apart of the shaft projecting beyondthe casing in one axial position ofthe piston is liable, in the other axial position of the piston, to bedrawn within the seal and thus further impair the sealingcharacteristics of the arrangement.

The imposed requirements of a small and slow movement are opposed to thevery desirable features of substantial piston movement and quickoperation. A substantial piston movement is quite necessary for a valvehaving large port apertures so that the apertures may be adequatelycleared to permit unrestricted flow through the valve and, since undervacuum conditions the flow rate through the valve is proportional to thecubic power of the aperture size, a large aperture is an importantrequirement for a vacuum valve.

One specific design to overcome shaft problems employs a bellows, oneend of which is sealed to the piston and the other end to the valvebody. The shaft then operates inside the sealed bellows unit and shaftleakage is avoided. Such bellows valves are well known and areextensively used in high vacuum work.

Suchv valves however have their own inherent disadvantages. The metalbellows material is very thin to obtain the necessary flexibility andshould it crack through fatigue effects or corrode away (if corrosivegases are being handled for instance) then once againa shaft leak wouldensue.

Methods of backing-up the bellows seal have been devised serving toillustrate that the shaft seal can still be of concern even in this typeof valve.

A more apparent disadvantage of the bellows type of valve lies in itsoperation. The piston is generally actuated by a screw threaded shaft sothat the sealwithin the valve may be made-firm and'locked. The operationof a screw-threaded shaft is necessarily slow and thus such valves,intrinsically, are not quick acting.

When such valves are made quick acting, as is generally done by means ofa cam or toggle mechanism to withdraw the shaft against some form ofspring tension, then the firmness of the internal seal depends entirelyupon the spring tension. Although now quick acting, such a valve cannotbe firmly tightened and is not locked.

The relevance of the above discussion is now apparent in that itillustrates that the previously stated objects of the valve inventionhereafter described are not met by, nor are mutually compatible with,existing forms of reciprocating piston vacuum valves.

.ln contrast to translational motion of a shaft through a seal,rotational motion of a shaft in a seal, without any accompanyingtranslation motionpcan be made leak-free for vacuum use.

This is because the previously mentioned first effect contributing toshaft leakage does not'arise-and the second effect is substantiallysmaller when the shaft merely undergoes a variably introduces a leakinto the valve due to a combination of two effects. Firstly, when theshaft passes through the seal a quantity of air which was absorbed inthe surface layers of the shaft is introduced into the valve. Thesesurface layers then out-gas" into the vacuum environment. No form ofseal which permits direct translational motion of the shaft from airinto the vacuum region (however well this seal may be devised) will beable to fully prevent this type ofleakage.

Secondly, when a shaft undergoes such translational movement a leakagemay occur as the contact between the seal and small rotation in the sealand is not moving through it.

In the valve hereafter described shaft leakage is effectively eliminatedas a cause of concern by virtue of the valve construction, whichrequires that the shaft seal is one which only involvesthe sealing of arelatively small and slow rotational motion, and by the form of the sealdesign and assemblage of components relating to the vacuum sealing ofthe shaft. The valve construction also incorporatesa tight internalvacuum sealing of the piston element to isolate the valve ports and alsoprovides a locked seal which is effected with a quick acting singleoperation. Other features relating to the previously stated objects ofthe invention will also become apparent from later description.

Two embodiments of the invention will now be described in detail by wayof example with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of the valve constituting a firstembodiment of the invention;

FIG. 2 is an external perspective view ofthe valve of FIG. 1;

FIGS. 3 and 4 are detail elevations of the spindle, operating means andhandle ofthe valve of FIGS. 1 and 2;

FIG. 5 is a detail cross-sectional view illustrating the vacuum seal forthe rotary spindle ofthe valve of FIGS. 1 to 4;

FIGS. 6 and 7 are detail views illustrating two stages in the assemblyof the annular sealing member on the end face ofthc piston element ofthevalve of FIGS. I to 5;

FIGS. 8', 9 and 10 are views illustrating the sealing operation of thesealing member of FIG. 8 when the piston element moves into one extremeposition, i.e. its scaling position;

FIG. II is a view similar to FIG. 8 but showing a flat, annular sealingmember mounted on the piston element in place of the ring shown in FIGS.6 to 10; and

FIG. 12 is a-view similar to FIG. I of a valve constituting a secondembodiment olthe invention.

Referring now to FIGS. I to I0 the valve casing is indicated generallyat I0 and comprises a piece of square section material II which isprovided with a bore l2 extending from one end I3 to an end wall 14 atthe other end thereof. the end wall 14 being provided with a first portl5 which is of lesser diameter than the bore 12, the port beingsurrounded by an annular seating surface 16 at the end of the bore 12.One of the walls of the member I] is provided with a second port 17which. as will be seen from FIG. 1, is placed below the annular seatingsurface 16.

The member Il may be metal although for special uses, which may forinstance involve the handling of corrosive gases. other suitablematerials such as plastics material may be used.

The material is deliberately chosen of square section because for asmall size ofvalve it enables the largest diameter of bore 12 to be usedand the largest diameter ofthe ports and 17 together with otherdesirable constructional features mentioned later.

The port ,15 is associated with a screw-threaded coupling 18 of knowntype provided with a flange 19 which is secured to the member 11 byscrews 20 and is vacuum sealed thereto by an O-ring 21 received in agroove in the flange 19. The port 17 is provided with a scrcw-threadedcoupling I8awhich is in all respects similar to the coupling 18. i

In a wall 22 of the member ll there is provided a cylindrical. outwardlyopening. recess 23 which communicates with the bore 12 through acylindrical aperture 24 of lesser diameter than the recess 23. Receivedin the recess 23 is a crank disc 25 which is a good fit in the recess soas to be rotatable therein and which is held in position by means of ashoulder 26 between the recess 23 and the aperture 24 and by a coverplate 27 which is removahly secured to the wall 22 by screws 28 andwhich is vacuum sealed to said wall by an O-ring 29 received in a grooveon the underside ofthe cover plate.

Referring particularly to FIGS. 3 and 4, the crank disc 25 has securedthereto an operating spindle 30 which is coaxial with the disc, thespindle being bored at 31 to receive the end of a handle 32, said endbeing held in position by a setscrew 33. The handle 32 is provided atits free end with an operating knob 34. The disc 25 is provided on itssurface remote from that connected to the spindle 30 with a crank pin 35which, as clearly shown in FIG. 3, is offset from the central axis 36ofthe disc 25 and the spindle 30 and has a portion 35cat the free endand which is of slightly larger diameter than the remainder of the pin.

Referring now to FIG. 5, the spindle 30 passes through a central bore 37in the cover plate 27. The bore is provided with an annular groove 38which is of wide, V-shape in cross section, the angle of the V beingapproximately 90 and the groove accommodates on Oring seal 39. The depthof the groove 38 is such that when the spindle 30 is inserted throughthe O-ring 39 the latter forms a tight vacuum seal around the spindle.

The part of the spindle extending within the seal may be speciallyfinished to form as perfect a cylindrical surface as practicable andthus have a high surface polish so as to provide optimum vacuum sealingof the said spindle.

The compression of the O-ring 39 results in it having three zones ofcontact with the cover plate and the spindle. Thus the ring has twozones of contact 41 with the cover plate along the sides of the grooveand has a third zone ofcontact 42 with the spindle. The two separatezones of contact 41 provide separate seals between the ring and thecover plate and also by their frictional contact with the cover plateact to prevent rotation of the ring within the groove 38 so that theonly rotational movement in the seal is between the smooth spindlesurface and the inner surface of the O-ring.

Preferably, the O-ring 39 is of comparatively hard material. such asneoprene or butyl rubber with a hardness of 50 to 60 Shore (durometer),and the O-ring 39 and the spindle lightly lubricated with vacuum greaseor a suitable vacuum oil ug. DC. 702 Silicone Oil, Dow Corning(orporation With the sealing member contained within a groove in thecover plate. the spindle may be inserted into its seal with minimum riskof damaging (Le. scratching or marking) the specially finished andpolished surface of the spindle. and hence there is little risk ofdamaging or impairing the vacuum seal during assembly ofthe componentparts.

To minimize the relative movement of the spindle and the sealing memberthe spindle 30 is made of comparatively small diameter since it is wellsupported by accurate location of the crank disc 25 in the recess 23 andby the transverse location of the crank disc between the shoulder 26 andthe cover plate 27. Due to this location of the spindle 30 by the crankdisc 25 there is little force tending to deform the O-ring 39 as thevalve is operated and this helps to maintain the effectiveness ofthcvacuum seal between the cover plate and the spindle 30. Moreover, thespindle 30 need only project a small distance from the cover plate 27 soas to reduce the transverse stresses on the spindle 30 and the O-ring 39as the valve is operated.

The arrangement is such that the spindle, the sealing member and thecover plate form a separate subassembly which can be positioned andremoved without disturbing the relative positions of the other parts ofthe valve or parts effect ing the vacuum sealing of the spindle. Thesubassembly also includes the operating means, the stop means, and ahandle or other means for rotating the spindle.

Referring now to FIG. I, a piston 42 is slidably mounted in the bore.12. After insertion of thepiston into the bore the latter is closed byan end plate 43 held in position by screws 44 and having a sealing ring45 to vacuum seal the end plate to the member 11. The piston 42 ismovable between two extreme positions in the bore 12 by means of thecrank pin 35 which is received in a slot 46 in the piston. The crank pinand the slot are made sufficiently long and deep respectively to provideadequate bearing surfaces between the pin and piston.

The bearing surface of the pin is on the portion 350 which has a lengthof approximately 30 percent of the total pin length 'lhe piston is thusdriven in the region of its axis and spinning motion which can arise dueto off-axis drive is removed from the seating motion of the piston.Also, to prevent spinning of the piston due to the uneven weightdistribution resulting from the presence of the slot 46. the piston ispro vided'with an off-axis hole 8 into which engages a guide pin 9mounted similarly off-axis and force-fitted into the end plate43/Spinning motion of the piston. which would be den-i mental to thevacuum sealing of the piston member. is thus prevented.

As spindle 30 is rotated the crank pin 35 will move across the slot 46and move the piston between its extreme positions. the pin driving thepiston directly without the use of intermediate linkages. When the crankpin is in the position indicated in dotted lines at 35a in FIG. 1, thepiston will be at the upper of its extreme positions and a sealing ring47 will engage the annular seating face 16 thus mutually isolating theports 15 and 17. It will be seen that when the crank disc is in thisposition, the crank pin 35 will be in a dead-center position; that is tosay the crank pin will be in the position shown in FIG. 3 so that a linethrough the pin and the axis 36 lies along the axis of the bore 12. Thepiston 42 will thus be locked in its upper extreme position and cannotbe moved except by rotation of the handle 32. The piston 42 cannot bemoved by pressure differences between the ends thereof because of thedead-center position ofthe crank pin 35.

The cover plate 27 is provided with stops 48 and 49 as shown in FIG. 2and when the handle 32 engages the stop 48 the piston 42 is in its upperextreme position as described above. The stop 48 is so located as toprevent any further rotation of the spindle or crank disc 25 over andabove that required to move the piston 42 to its upper extreme positionand to cause sealing engagement between the ring 47 and the annularseating surface 16 as will hereinafter be described. If the handle 32 isnow rotated in an anticlockwise direction in FIG. 2 until it reaches thestop 49, the piston 42 will be in its lower cxtrcmc position and thecrank pin 35 will be in the position shown in dotted lilies at 35b inFIG. I. In this position, also, the crank pin 35 will be in a deadcentcrposition and therefore the only way in which the piston can be moved isby rotating the handle 32, the piston cannot be moved by differences inpressure across its ends. It will be seen that the valve is positivelylocked when in either of its closed or fully open positions.

Referring now to FIGS. 6 and 7, the O-ring 47 is retained in a groove 50in the end face of the piston and formed between a peripheral rib S1 anda central spigot 52 whose periphery is under cut as clearly shown in theFIGS. The depth of the groove is approximately 80 percent of thesectional diameter of the O-ring. After the O-ring has been placed inthe groove as shown in FIG. 6, the peripheral rib 51 is folded inwardlyas shown in FIG. 7 to trap the O-ring in the groove. The rib isinitially ofa height which is greater than the diameter of the O- ring47 so that as it is folded inwardly as shown in FIG. 7 it does not tendto bite into the O-ring but merely holds the O- ring in position andprevents it from being withdrawn by suction effects within the valve.The resulting groove is of generally trapezoidal or wedge section andthe O ring 47 initially projects from the groove as shown in FIG. 7.

The sealing operation ofthe O-ring is illustrated in FIGS. 8, 9 and 10.Since the O-ring initially projects from the end face of the piston, asthe piston is moved to its upper extreme position, i.e. as the crank pin35 approaches the position 35a, the O-ring will contact the annularseating 16 as shown in FIG. 8. As the piston continues to move to theright in FIG. 8, the O- ring becomes deformed and begins to spread intothe groove 50 as shown in FIG. 9. At the end of the sealing operation,the O-ring substantially fills the groove 50. The trapezoidal or wedgesection of the groove 50 allows adequate room for movement of the O-ringmaterial as the O-ring is compressed as shown in FIGS. 8 to so that theelastic properties ofthe O-ring are not impaired and it therefore has along operational life. Since the forward motion ofthe piston is limitedto an end position defined by the dead-center position ofthe crank pin,the O-ring compression does not exceed a predetermined design value.

A tight scaling is highly desirable in a vacuum valve and in the presentarrangement. where a screw operation is not being used. there is alimitation upon the force that can be exerted by the piston to provide atight seal. To obtain a high sealing pressure it is therefore necessaryto use a narrow sealing ring 47 in contrast to the use ofthick rings orlarge area gaskets. As the piston is a sliding fit in the bore ofthevalve and piston motion is well defined, thin rings or gaskets can besatisfactorily employed.

Taking a specific example, if the O-ring 47 has a diameter ofthree'fourth inch and sectional thickness ofone-sixtcenth inch there isring contact as indicated at 53 in FIG. 10 over an area ofone-tenthsquare inch.

In the specific embodiment described, the mechanical advantage of theoperating mechanism of the valve is approximately 7:1 for a convenientlyshort length ofopcrating handle so that, even for a manually appliedforce of some l0 ounces at the handle, the scaling pressure between theO-ring 47 and the annular seating surface 16 will nevertheless beapproximately 40 psi. This pressure is sufficient to produce acompression of more than 30 percent cross section of the above 0- ringeven for the maximum hardness of O-rings normally supplied commerciallyShore).

It is apparent that to utilize the pressure at the seal obtainable withthis valve construction it is preferred to use an O-ring of at least 60Shore hardness in the piston end face. A tight seal is thereforeobtained which is comparable to that achieved in valves utilizingscrew-threaded operating means and which have the disadvantage of beingextremely slow in operation as compared with the present arrangement.

The use of a narrow ring of flat gasketmaterial, though also possible,is restricted in general by the lack ofeasy entrapment in the piston endface to provide retention yet allow room for elastic deformation.Specially moulded rings for this purpose have no advantage as such overthe commercially available torroidal rings (O-rings) for which a simpleentrapment as described above can be used.

In this connection however, an arrangement using flat gasket materialfor a sealing member and which has proved very satisfactory with thevalve herein described is shown in FIG. 1.1 in which the central regionof an annular flat gasket is secured rigidly in an annular groove 54 inthe end face of the piston by means of an end cap button 55 the shank ofwhich is force-fitted into an axially bored hole 56 in the piston endface. The flat gasket material is thus retained in the piston end face,the central region is inert, and the outer annulus 57 of the gasket thencomprises the required narrow sealing ring. On sealing, the narrowannular ring section undergoes elastic deformation into the providedspace 58 at the outer edge of the ring. The inner edge 59 of the seatingsurface is rounded to prevent shearing of the gasket material when theseal is made.

The provision of the detachable end plate 43 and the cover plate 27readily enables the piston 42 to be removed from the valve body formaintenance or replacement without requiring the removal of the valvebody itself from any equipmentto which it may be connected by thecouplings 18 and 18a. Furthermore, the cover plate 27 with the spindle30, handle 32, crank disc 25 and crank pin 35 form a subassembly whichmay easily be removed and replaced without having to disturb the partseffecting the vacuum scaling of the spindle 30.

Referring now to FIG. 12, this shows a second embodiment ofthe'invention. This second embodiment differs from the first embodimentsolely in the form of the piston and in the ar rangement whereby thepiston can seal the first port in the valve casing. Parts in FIG. 12which are identical to corresponding parts in FIGS. 1 to 11 areindicated by the reference numerals used in FIGS. I to 11 with theprefix 1.

Referring to FIG. 12, the valve casing 60 comprises a piece 60a ofsquare section material which has a bore 61 extending from end to endthereof. One end of the bore is closed by an end plate 143 held inposition by screws 144 and vacuum sealed to the end of the casing by anO-ring seal 145.The other end of the bore 61 is provided with acounterbore 62 and is furnished with an inserted sleeve 63. The sleeve63 is vacuum sealed to the casing 60 by means of an O-ring seal 64inserted between opposed shoulders on the casing and the sleeve.

The sleeve 63 has a cylindrical bore 65 and at its inner end this borediverges at 66 to provide a convergent entry into the bore 65 from thebore 61 of the valve. The bore 65 provides an annular seating surface aswill be described and also a first port 67 of the valve. The sleeve 63is held in position by means ofthe coupling 118 which is held inposition in turn on the easing by screws 120 and is vacuum sealed to thecasing by means ofan O-ring seal 121.

A piston 68 is slidable in the bore 61 and is provided with a' slot 146in which is received the crank pin of operating mechanism which issubstantially identical to that shown in FIGS. I to 10.

The piston 68 has a stem 69 in which is formed a peripheral groove whichcarries an O-ring seal 70. When the O-ring 70 is uncompressed it has adiameter which is slightly greater than the diameter of the bore 65 inthe sleeve. The piston may be moved to close the valve by rotating thehandle [32 and thus moving the piston 68 as described in relation toFIGS. 1 to until the O-ring seal 70 enters the bore 65. The entry oftheO- ring is facilitated by the diverging portion 66 of the bore and whenthe O-ring 70 is fully in the bore 65 it is radially compressed andprovides a vacuum seal for the port 67. When the handle 132 is turned towithdraw the O-ring 70 from the bore 65 there is communication betweenthe first port 67 and the second port 117. As in the previous embodimentthe valve is locked when in the fully closed or fully open positions.

For the embodiment to provide a good vacuum seal there are severalnecessary requirements. The first of these is that the O-ring 70 must hemade of a material which is sufficiently hard that is does not deform orcxtrudc from its annular groove as the O-ring is forced into the bore65. The approximatcly 7:l mechanical advantage of the operatingmechanism readily permits a reasonably hard O-ring to be forced withinthe bore 65 and radially compressed and thus it is preferred to use forthe material of the ()-ring a neoprene or hutyl rubber of at least toShore (duromctcr) hardness. the ()ring being lightly lubricated withvacuum grease.

Secondly. the bore ofthe sleeve must have a smooth and high qualityfinish. The fact that the sleeve is removable front the casing makes itcomparatively easy to obtain such a finish and also to obtain thedivergent portion 66. The sleeve may be made from any desired materialand particularly from a material which is different from that of thevalve casing.

Thirdly. as frictional wear is the usual limitation on the satisfactoryuse of O-rings for such sliding seals. and since the frictional wearincreases with the O-ring hardness. it is necessary to make the sleevefrom a material having low frictional properties in relation to theO-ring 70.

A preferred material for the sleeve is thus nylon or a material such asTeflon or P.T.F.E. (Registered Trade Marks for polytetrafluroroethylene)which has extremely low frictional properties and which, by also havinga very low vapor pressure. is a very satisfactory material for vacuumuse. With an insertable sleeve constructed of this material the wear anddamage to the O-ring is minimal.

The fact that the sleeve 63 is readily detachable from the valve casingmeans, additionally, that it may readily be replaced should it requiredservice.

The sealing arrangement for the port 67 in the embodiment of FIG. 12differs from the corresponding sealing arrangement ofthe valve of FlGS.l to 11 in that it is not capable ofeffecting such a tight, static.compression seal. The advantage ofthe second embodiment, employing asliding seal, is that the machining accuracy of the piston and thelength of the valve bore is rendered substantially less important and itis also not necessary to prevent possible spinning motion of the pistonas in the first embodiment.

Since there is a reasonable mechanical advantage of the operatingmechanism it is still possible to obtain adequate radial compression ofthe desired form of O-ring 70 to effect a good tight vacuum seal.

It will be seen that the invention provides simple fast-acting valveswhich are useful in high-vacuum systems and which have the advantagesset forth above over previously used vacuum valves.

lclaim:

1. A valve comprising:

a. a casing having a bore and first and second ports which communicatewith the bore, the first port communicating with the bore at an endthereof;

b. a piston element slidable within the bore between two extremcpositions in one of which it mutually isolates the ports and in theother of which it allows the ports to communicatc through the bore;

c. an annular seating surface surrounding the first port;

d. a resilient. deformable. annular sealing member housed in an annulargroove in the piston element. the member. when uncompressed, projectingbeyond the mouth of the groove for sealing engagement with said seatingsurfaces;

e. a valve-operating spindle extending through. and mounted forrotational movement in. a wall of the casing and having an outer partprojecting outwardly of the easing;

f. a crank disk extension of the spindle. located in a recess in thewall ofthe casing;

g. operating means within the casing connecting the crank disc and thepiston element to effect reciprocating motion of the piston element inthe bore between said extreme positions when the spindle is rotated andsuch that when the piston element is in either of said extreme positionsit cannot be displaced by pressures in the bore acting on it. saidscaling member being compressed by. and scalingly engaging. said seatingsurface when the piston element is in said one extreme position;

h. stop means for limiting the rotational movement of the spindle tothat required to displace the piston element between said two extremepositions;

i. a cover plate secured to the casing and through which the spindleextends; and

j. a sealing ring held against rotational movement and interposedbetween the cover plate and the spindle to provide a seal between thecover plate and the spindle.

2. A valve according to claim 1 wherein the operating means comprises acrank pin on the crank disc and wherein the stop means serves to limitrotation ofthe operating spindle to rotation between two dead-centerpositions of the crank pin. whichdead-ccnter positions correspond to thetwo extreme positions ofthe piston element.

3. A valve according to claim 1 where the crank disc is located in saidrecess between a shoulder in the casing wall and said cover plate whichis removably secured to the valve casing.

4. A valve according to claim 2. wherein the crank pin is received in aslot in the piston element.

5. A valve according to claim I wherein the operating spindle passesthrough a bore in the cover plate and wherein said sealing ring is anO-ring seal received in a V-shaped groove in said bore and engaging theoperating spindle so that the O-ring is deformed to generally triangularsection and has a single zone of contact with the spindle and twoseparate zones of contact with the sides of the groove.

6. A valve according to claim 1 wherein the outer part of the spindlecarries an operating handle and wherein the stop means comprises twopins mounted on the cover plate and arranged to be engaged by the handleat said extreme positions ofthe piston element.

7. A valve according to claim 1 wherein the cover plate and crank discform a detachable subasscmbly which comprises the operating means andstop means and the vacuum seal for the spindle.

8. A valve according to claim 1 wherein the annular seating surface isdirected to an end face of the piston element and wherein the annulargroove carrying the annular sealing member is in said end face.

9. A valve according to claim 1 wherein said annular groove is ofgenerally trapezoidal section to retain the annular sealing member andis substantially filled by said sealing memberwhen the latter iscompressed by engagement with the seating surface when the pistonelement is in its one extreme position.

10. A valve according to claim 1 wherein the annular seat ing surface iscylindrical and wherein the annular sealing member enters thecylindrical seating surface when the piston is in sald one extremeposition.

11. A valve according to claim 1 including a sleeve insert mounted inthe casing and vacuum sealed thereto. the bore of the sleeve insertproviding the first port and the annular seating surface.

11 A valve according to claim 1 wherein the end of said bore remote fromthe first port is closed by a removable end plate.

